Method of pressure treating electrophotographic recording elements to change their sensitivity to light

ABSTRACT

THE SENSITIVITY OF AN ELECTROPHOTOGRAPHIC RECORDING ELEMENT, COMPRISING A SUBSTRATE AND A PHOTOCONDUCTIVE LAYER THEREON IS CHANGED BY A METHOD WHEREIN PRESSURE IS APPLIED ACROSS THE RECORDING ELEMENT WITH SUFFICIENT FORCE TO COMPRESS IT, THE RESULTING SENSITIVITY BEING A FUNCTION OF THE APPLIED PRESSURE. ELECTROPHOTOGRAPHIC RECORDING ELEMENTS, CAPABLE OF PROVIDING HALF-TONE PRINTS, COMPRISE A PATTERN OF A HALF-TONE SCREEN EITHER EMBOSSED OR IMPRESSED BY PRESSURE, ON A PORTION OF THE PHOTOCONDUCTIVE LAYER.

I N VEYTUR Philip J. Donald ATTORNEY Aug. 1, 1972 P.J DONALD METHOD OF PRESSURE TREATING ELECTROPHOTOGRAPHIC RECORDING ELEMENTS TO CHANGE THEIR SENSITIVITY TO LIGHT Filed Jan. 2, 1970 United States Patent 3,681,071 METHOD OF PRESSURE TREATING ELECTRO- PHOTOGRAPHIC RECORDING ELEMENTS TO CHANGE THEIR SENSITIVITY T0 LIGHT Philip Joseph Donald, Woodbury, N.J., assignor to RCA Corporation Filed Jan. 2, 1970, Ser. No. 202 Int. Cl. G03g /08 US. Cl. 96-1.8 4 Claims ABSTRACT OF THE DISCLOSURE The sensitivity of an electrophotographic recording element, comprising a substrate and a photoconductive layer thereon, is changed by a method wherein pressure is applied across the recording element with sufiicient force to compress it, the resulting sensitivity being a function of the applied pressure. Electrophotographic recording elements, capable of providing half-tone prints, comprise a pattern of a half-tone screen either embossed or impressed by pressure, on a portion of the photoconductive layer.

BACKGROUND OF THE INVENTION This invention relates generally to electrophotographic recording elements, and more particularly to a method of treating an electrophotographic recording element, to change its sensitivity to light, and to electrophotographic recording elements so treated. The electrophotographic recording elements produced by the novel method are particularly useful for copying continuous-tone (continuously variable shades of gray) images by electrophotographic processes using thin sheets of paper coated with a thin layer of a photoconductor.

Most electrophotographic recording elements have relatively short gray scales and, by themselves, reproduce continuous-tone images poorly. It has been proposed to reproduce a continuous-tone image on an electrophotographic recording elemeut by employing an external halftone screen during the exposure operation in an electrophotographic process. A disadvantage of this prior-art procedure is that the optical system employed for projecting the image imposes .a limitation on the resolution and the number of resolvable elements to be copied. It has also been proposed to provide a special electrophotographic plate for producing a half-tone image. The latter electrophotographic plate comprises a metal backing plate formed with a pattern of alternate projections and depressions and filled with a photoconductive material. While such an electrophotographic plate may be suitable for certain applications, the prior-art electrophotographic plate and its method of manufacture leaves much to be desired for making direct copies in electrophotographic processes that employ electrophotographic recording elements of thin paper coated with a thin layer of a photoconductor, such as zinc oxide.

The novel method of treating electrophotographic recording elements and the novel electrophotographic recording elements produced thereby provide means for copying continuous-tone images and obviate the aforementioned disadvantages of the prior-art methods and recording elements. Also, the novel electrophotographic recording elements exhibit an extended gray scale characteristic that is not evident in prior-art untreated electrophotographic elements.

SUMMARY OF THE INVENTION The novel method of treating an electrophotographic recording element, comprising a substrate and a photoconductive layer thereon, to change its response to light, comprises applying pressure between the substrate and the layer to compress it. The resulting change is sensitivity of the recording element to light is a function of the pressure applied to compress the recording element. In one embodiment of the novel method pressure is applied to either emboss or impress a half-tone screen on one of the major surfaces of the recording element. The novel electrophotographic recording elements produced by the novel method have altered sensitivity to light in those portions where they have been compressed.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a plan view of an electrophotographic recording element, one half of which has been treated by the novel method of the present invention, showing an image developed simultaneously on both halves, to illustrate the effect of the novel method;

FIG. 2 is a schematic drawing of apparatus for applying pressure to an electrophotographic recording element in accordance with one embodiment of the novel method;

FIG. 3 is a plan view, with a portion in phantom, of a photographic, step-wedge gradient, light filter (transparency) used to expose the recording element illustrated in FIG. 1;

FIG. 4 is a fragmentary plan view of an embodiment of a novel electrophotographic recording element produced by apparatus illustrated in FIG. 5;

FIGS. 5 and 5a are schematic drawings of apparatus for treating an electrophotographic recording element in accordance with another embodiment of the novel process;

FIG. 6 is a fragmentary plan view of another embodiment of the novel electrophotographic recording element produced by the apparatus illustrated in FIG. 8;

FIG. 7 is a fragmentary cross-sectional view of the recording element illustrated in FIG. 6, taken along the line 77, and showing a template in phantom in the process of removal from the recording element;

FIG. 8 is a schematic diagram of apparatus for treating an electrophotographic recording element in accordance with still another embodiment of the novel method; and

FIG. 9 is a fragmentary perspective view of a template used with the apparatus shown in FIG. 8 to produce the novel electrophotographic recording element illustrated in FIGS. 6 and 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 of the drawing, there is shown a developed electrophotographic recording element 10 divided into two substantially equal parts, one part 12 being untreated (non-pressurized, prior-art) and the other part 14 being treated (pressurized) by the novel method of the present invention. The pressurized part 14 is illustrative of one embodiment of the novel electrophotographic recording element. The recording element :10 comprises a relatively conductive substrate 16 (FIG. 2), such as paper, and a photoconductive layer 18, such as photoconductive particulate zinc oxide in a suitable resin binder. The non-pressurized part 12 of the recording element 10 represents prior-art.

The pressurized part 14 of the recording element 10 has been treated by the novel method to compress it, as by passing one half (part 14) of the recording element 10 between a pair of smooth parallel pressure rollers 20 and 22 (FIG. 2). The rollers 20 and 22 are rotated by any suitable means (not shown) in the directions indicated by the curved arrows associated with the rollers. The roller 20 is urged against the roller 22 by any suitable force means, such as springs (not shown), in the directions of the straight arrows 24 and 26 to provide a pressure sufiicient to compress the photoconductive layer 18 of the recording element 10.

It has been found that for a recording element comprising a substrate 16 of paper and a photoconductive layer of zinc oxide in a resin binder, a pressure of at least 50 p.s.i. is necessary to compress the photoconductive layer 18. Pressure upon the recording element 10 of sufiicient force to compress the photoconductive layer 18 reduces the photoconductive response of photoconductive layer 18 to light, and this reduction is a function of the amount of pressure applied to the recording element. Recording elements 10 of the type described have been pressurized with pressures of between 50 p.s.i. and 10,000 p.s.i. to provide recording elements of successively decreasing sensitivity, as may be desired in an electrophotographic printing process. It has also been found that the application of pressure to a recording element 10 not only changes its sensitivity but also increases its gray scale.

The aforementioned results are illustrated in the exposed and developed recording element 10 illustrated in FIG. 1. The pressurized part 14 of the recording element 10 was treated by subjecting it to a pressure of about 5,000 p.s.i., and the non-pressurized part 12 was untreated. Both parts 12 and 14 of the recording element 10 were then uniformly charged (in the dark) with a negative electrostatic charge by a corona discharge device, in a manner well known in the art, and then (both parts 12 and 14) were simultaneously exposed to light through a photographic, step-wedge gradient, light filter 28, shown in FIG. 3. The exposed recording element 10 was then developed with an electroscopic liquid toner and it provided the step-wedge images illustrated in FIG. 1.

The filter 28 is a transparency, such as of glass of photographic plastic film, comprising a plurality of successive steps of light transmitting material, ranging from a black step 30, through successively more lighttransrnitting steps of gray 31-36, to a clear transparent step 37.

A comparison of the developed images on the treated pressurized part 14 and the untreated non-pressurized part 12 of the recording element 10 indicates that pressure on the part 114 both reduced its sensitivity to light and increased its gray scale with respect to the untreated part 12. For example, light passing through the step 31 of the filter 28 exposed the untreated part 12 to form a step image 31a, but the same light did not expose the pressurized part 14, producing a black (unexposed) step image 3112. On the other hand, the length of the gray scale reproduced on the untreated part 12 was relatively smaller than the length of the gray scale reproduced on the pressurized part 14. For example, the simultaneous exposure of the filter 28 on both of the parts 12 and 14 produced a developed gray scale on the untreated part 12 that ranged from black to white in three steps 31a, 32a, and 33a of gray, whereas the developed gray scale on the pressurized part 14 was shifted and ranged from black to white in five steps 31b, 32b, 33b, 34b, and 35b of gray. Hence, although the photoconductivity of the photoconductive layer 18 is reduced in the pressurized part 14, it acquires a longer gray scale, making the pressurized recording elements more suitable for copying continuous-tone images. Recording elements that have been pressurized with forces ranging between 50 to 10,000 psi. provide a range of copying materials for reproducing images of different contrast in an electrophotographic process in a manner analogous to the range of photographic printing papers of difierent sensitivity for reproducing images of different contrast in a silver halide photographic process.

Referring now to FIG. 4, there is shown another novel recording element 40 pressurized in accordance with the novel method. Only portions of the recording element 40 are pressurized to provide it with either an impressed or embossed pattern of a photographic half-tone screen. The pattern of the half-tone screen is applied to the recording element by pressure means, such as by moving a recording element of the type described in the aforementioned patents between pressure rollers 42 and 44 (FIG. 5). A pressure of between 50 and 10,000 p.s.i. is applied to the recording element 40 by pressure applied to the rollers 42 and 44, by any means in the art, in the direction of the arrows 43 and 45. The roller 44 is smooth and is shown adjacent the substrate 16 of the recording element 40 in FIG. 5. The photoconductive layer 18 is adjacent the roller 42, the roller 42 being formed with a pattern of a plurality of projections adapted to compress the photoconductive layer 18 to provide thereon compressed portions 46 in the pattern of a half-tone screen. The compressed portions 46 are spaced to provide a half-tone screen pattern equivalent to between 50 and 250 lines per inch and may cover between 50% and of the upper major surface 48 of the recording element 40.

It is also within the contemplation of the present invention to compress a portion of the photoconductive layer 18 in such a manner so that the pattern of the halftone screen is embossed, that is raised. It is only necessary that the half-tone screen pattern, either embossed or impressed, has a dilferent sensitivity to light from the remainder of the photoconductive layer 18 to provide halftone copies of continuous-tone images.

It is also 'within the contemplation of the present invention to apply pressure to the recording element 40 with the photoconductive layer 18 adjacent the smooth roller 44 and with the substrate 16 adjacent to the patterned roller 42, whereby a pattern of a half-tone screen can be impressed or embossed on the inner (lower) major surface 50 of the photoconductive layer 18 by pressure transferred through the relatively soft paper substrate 16, as shown in FIG. 5a. In the latter method, the upper major surface 48 of the photoconductive layer remains smooth, and the lower major surface 50 has the pattern of the half-tone screen either embossed or impressed thereon.

The novel electrophotographic recording element 40 is capable of providing a better electrophotographic copy of continuous-tone images than a prior-art, non-pressurized, untreated recording element (such as part 12 of the recording element 10) because the novel recording element 40 not only has its original gray scale in its unpressurized portions but also its increased and shifted gray scale in its pressurized portions. Since the gray scales of both the pressurized and unpressurized portions of the novel recording element 40 are intermingled with each other on the photoconductive layer 18, the recording element 40 functions effectively as having a single extended gray scale equivalent to the combined gray scales of the pressurized and unpressurized portions.

Referring now to FIG. '6, there is shown another novel recording element 60, also comprising the substrate 16 and the photoconductive layer 18 as described for the recording element 10. In the recording element 60 the photoconductive layer 18 is formed with a plurality of raised portions 62 by compressing the recording element with a template 64 (FIG. 9) between a pair of parallel smooth pressure rollers 66 and 68 (FIG. 8). The template 64 is formed with a plurality of holes 70 therein in the pattern of a half-tone screen. It is placed over the photoconductive layer 18 of the recording element 60 and both are moved between the pressure rollers 66 and 68 in a direction indicated by the curved arrows associated with them. Pressure of between 50 and 10,000 p.s.i. is applied in the direction of the straight arrows 72 and 74 to produce the embossed pattern of a half-tone screen, provided by the raised portions 62. It is also within the contemplation of the present invention for the template 64 to be placed adjacent the substrate 16 of the recording element 60 and moved between the pressure rollers 66 and 68 so as to emboss or impress a half-tone pattern on the inner major surface 50 of the photoconductive layer 18, leaving the outer surface 48 smooth. Regardless, however, of the surface of the photoconductive layer 18 on which the pattern of the half-tone screen is impressed or embossed, the weight per unit area of the novel electrophotographic recording element remains substantially the same.

Although the novel electrophotographic recording elements have been described as being treated by the novel method of applying pressure across them with rollers, it is within the contemplation of the present invention to apply this pressure by any other suitable means known in the art. For example, the pressure may be applied by a hydraulic press, and one of the pressure plates of the press can be engraved to impress or emboss a pattern of a half-tone screen on one of the major surfaces of the photoconductive layer.

'Some of the novel (pressurized) electrophotographic recording elements may have substantially the same appearance to the naked eye as that of the prior-art untreated electrophotographic recording elements. This is especially true where the applied pressure to untreated recording elements 'was relatively slight or where Whole sheets of prior-art recording elements have been subjected to pressure between their entire major surfaces. The structure of the novel recording element, however, differs from the untreated (unpressurized) recording element in that the portions of the photoconductive layer of the novel recording element have a greater density where they have been compressed than any portions of the untreated (unpressurized) recording element. Also, the density, that is the weight per unit volume, of the compressed portions of the novel recording element, resulting from the applied pressure to compress the portions, is substantially uniform throughout the thickness of the photoconductive layer between its major surfaces.

I claim: 1. In a method of treating an electrophotographic recording element to change the response of at least a portion thereof to light, said recording element comprising a substrate, a continuous uniformly thick photoconductive layer on said substrate, and opposite substantially planar, parallel, major surfaces, one of said major surfaces being that of said photoconductive layer, and the other of said major surfaces being that of said substrate, said method comprising applying between 50 p.s.i. and 10,000 p.s.i. pressure between said major surfaces of said recording element in said portion thereof to compress the thickness of said portion so that said one major surface remains planar but said other major surface is depressed at said portion, whereby to change the sensitivity to light of said portion with respect to the sensitivity to light of the remainder of said recording element, said layer comprises a photoconductor of particulate zinc oxide in a resin binder, and

said pressure is applied to compress said portion of said layer to provide a pattern of a half-tone screen embossed or impressed only in its surface adjacent said substrate, said one major surface remaining planar.

2. In an electrophotographic recording element of the type comprising a substrate and a continuous photoconductive layer of particulate zinc oxide in a resin binder thereon, the improvement comprising a portion of said layer, defining a pattern of a halftone screen of between 50 and 250 lines per inch and between 10% and of said recording element, being compressed,

said pattern being compressed in said substrate and the surface of said layer adjacent said substrate, the surface of said layer remote from said substrate being smooth,

said portion having a different response to light of a given intensity than the remainder of said layer, and said portion and said remainder each having substantially the same weight per unit area.

3. In an electrophotographic recording element of the type comprising a substrate and a continuous photoconductive layer of particulate zinc oxide in a resin binder thereon, the improvement comprising a portion of said layer and said substrate being compressed, but the entire surface of said layer remote from said substrate being smooth,

said portion of said layer having a diflerent response to light of a given intensity than the remainder of said layer,

said portion and said remainder each having substan tially the same weight per unit area, and

said remainder comprising a pattern of a half-tone screen of between 50 and 250 lines per inch and 10% and 90% of said recording element.

4. A method of treating an electrophotographic recording element to change the response of at least a portion thereof to light, said recording element comprising a substrate, a continuous uniformly thick photoconductive layer of particulate zinc oxide in a resin binder thereon, and opposite major surfaces, said method comprising:

applying at least 50 p.s.i. pressure between said major surfaces of said recording element at said portion thereof, with means having a smooth surface disposed adjacent said layer and a raised pattern of a half-tone screen adjacent said substrate, to compress the thickness of said portion and to provide said pattern of a half-tone screen, equivalent to between 50 and 250 lines per inch, embossed or impressed in one of said major surfaces adjacent said substrate, said pattern covering between 10% and 90% of said major surface, and the other of said major surfaces adjacent said layer remaining smooth.

References Cited UNITED STATES PATENTS 2/1971 Weigl 96l R 9/1965 Gold 117--17.5X

OTHER REFERENCES GEORGE F. LESMES, Primary Examiner R. E. MARTIN, Assistant Examiner US. Cl. X.R. 96l.5 

